A superconductor with unique properties such as (1) having zero electric resistance, (2) being perfect diamagnet and (3) exhibiting the Josephson effect, is expected to have wide applications including transmission of electric power, generation of electric power, confinement of plasma for nuclear fusion, magnetically levitated vehicles, magnetic shields, ultra-high sensitivity magnetic sensors, high-speed microwave communication devices and high-speed computers.
In the past, known superconductors were only those having low superconducting transition temperatures (Tc) such as NbTi and Nb3Sn. In 1986, Bednorz and Mueller discovered a copper oxide high temperature superconductor (La1-xBax)2CuO4 that has a superconducting transition temperature (Tc) of about 30K (J. G. Bednorz and K. A. Mueller: Z. Phys. B64, 189 (1986)).
This report was followed by successive reports of oxide high temperature superconductors having ever higher superconducting transition temperatures (H-Tc) such as YBa2Cu3Oy (Tc=90K), Bi2Sr2Ca2Cu3Oy (Tc=110K), Tl2Ba2Ca2Cu3Oy (Tc=125K), HgBa2Ca2Cu3Oy (Tc=135K). Many researches have been conducted and published on the manufacturing method, physical properties and applications of these materials. Among these materials, YBa2Cu3Oy is a high temperature superconductor that does not include harmful substances such as Tl and Hg and has relatively low anisotropy, and is therefore viewed as the most promising candidate for a material to be used in practical applications such as electronic devices and electric wires. It is known that the most useful superconducting properties can be obtained when the valence n of copper atom (Cun+) in the formula is within a range of 2.0<n<2.67, each superconductor having a sufficient number of oxygen atoms to meet the requirements of a the valence of copper.
The YBa2Cu3Oy superconductors is known to exhibit superconducting transition temperature Tc of about 90K also when the component Y is replaced with a rare earth element (La, Nd, Sm and Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) (superconductors obtained from such substitution are collectively called the 123 superconductor.)
Applications of superconductor to electronic devices include a Josephson device that utilizes the Josephson effect, that requires the Josephson junction forming technology that employs physical film forming process such as sputtering or laser deposition process to form a thin film. For the Josephson junction of an oxide high temperature superconductor, various forms have been proposed including bi-crystal junctions, bi-epitaxial junctions, step-edge junctions, bridge junctions, ramp-edge junctions and stacked junctions (Susumu Takada, Oyo Butsuri 62, 443 (1993)). Among these, the ramp-edge junction that has a tunneling barrier layer formed obliquely between a pair of oxide high temperature superconductors, in particular, is viewed as a promising junction structure because the tunneling barrier layer has a high driving capability during switching operation and the critical current can be controlled by adjusting the thickness of the tunneling barrier layer (Mutsuo HIDAKA, Tetsuro SATO, Shuichi TAHARA: Oyo Butsuri 67, 1167 (1998)).
FIG. 1 schematically shows the ramp-edge type Josephson junction.
The ramp-edge type Josephson junction is formed by depositing a high temperature superconductor thin film 2 as a first superconductor on a substrate 1 by a physical film forming process, forming an inter-layer insulation film 3 such as CeO2 or SrTiO3 on the high temperature superconductor thin film, etching the high temperature superconductor thin film 2 together with the inter-layer insulation film so as to form an edge, forming a layer that would make a barrier on the edge surface 2a so as to form a junction, and depositing a high temperature superconductor thin film 4 as a second superconductor thereon.
Indicators that represent the performance of a Josephson junction include IcRn product. The IcRn product is the product of the maximum current (critical current Ic), that can flow under superconducting condition at a given temperature, and the resistivity (Rn) of the junction that has returned to the state of normal conductivity upon disruption of superconductivity, the product being normalized by the dimension of the junction. Qualitatively, the IcRn product represents the magnitude of signal handled during switching. The larger the IcRn product, the higher the speed of switching operation can be made.
The ramp-edge junction shows larger value of IcRn product than the Josephson junctions of other structures. For example, such a ramp-edge junction has been proposed that PrBa2Cu3Oy (PBCO) is used for the tunneling barrier layer and YBa2Cu3Oy (YBCO) is used for the high-temperature superconductor that makes the upper and lower electrodes.
The stacked junction is made by stacking an oxide high temperature superconductor layer, a tunneling barrier layer and an oxide high temperature superconductor layer one on another, and has been researched for the prospect of being advantageously used in building large-scale integration in the future.
FIG. 2 is a sectional view of a stacked type high temperature superconducting Josephson junction. The stacked type high temperature superconducting Josephson junction is made by forming a first superconductor 2 and an inter-layer insulation film 3 successively on a substrate 1, forming an opening 3b in the inter-layer insulation film 3 at a position where a Josephson junction is to be made, making a junction by forming a barrier layer in the opening 3b, and depositing a second superconductor 4 thereon.
In the ramp-edge junction and the stacked junction, PrBa2Cu3Oy (PBCO) layer, Nb-doped SrTiO3 layer or a process-damaged layer is used as the tunneling barrier layer.
The tunneling barrier formed in the form of a thin film is referred to as an artificial barrier. The tunneling barrier that utilizes, without forming a thin film, a layer of which the surface has been damaged by the bombardment of ions during the process is referred to as Interface-Engineered Junction (IEJ).
The IEJ has such an advantage that larger tunnel current can be drawn because of an extremely thin junction layer, and therefore has been researched extensively (B. H. Moeckly et al., Appl. Phys. Lett. 71, 2526 (1997)).
In the IEJ, existence of an extremely thin layer having thickness of around 1 to 2 nm formed in the junction has been confirmed through observation with a transmission electron microscope (TEM). This extremely thin layer is believed to function as the Josephson junction, although detailed structure thereof is yet to be established (for example, J. G. Wen et al., “Advances in Superconductivity XII”-Proc.ISS' 99, p. 984, (Oct. 17–19, 1999), Y. Soutome et al., “Advances in Superconductivity XII”-Proc.ISS' 99, p. 990, (Oct. 17–19, 1999)).
A first problem encountered in the prior art will now be described below. Recently a schematic diagram of the junction structure based on observation as shown in FIG. 3 has been published (Y. Soutome et al., “Advances in Superconductivity XII”-Proc. ISS' 99 (Oct. 17–19, 1999, Morioka) P. 990)
The diagram shows that the intervals between CuO2 planes, that are major transmission paths for high temperature superconduction, in the c-axis direction are the same in the first and second superconductors since the junction is formed by using the same high temperature superconductor for the first and second superconductors. As a result, it can easily be anticipated that failure to satisfactorily perform as a Josephson junction may occur frequently as the junction layer becomes unacceptably thinner due to minor fluctuations in the process conditions during the formation of the junction that is carried out under very sensitive conditions, resulting in short circuiting in the junction. This problem will be described below with reference to FIGS. 4A through 4C.
FIGS. 4A through 4C illustrate the current-voltage (I-V) characteristic across the junction varying with different thickness of the Josephson junctions.
When the junction layer is too thick, superconducting current cannot flow as shown in FIG. 4A.
When the junction layer has a proper thickness, superconducting current can flow by tunneling through the junction without causing a voltage within the range of critical current Ic, as shown in FIG. 4B. As soon as the current exceeds the critical current Ic, a voltage appears across the junction. The I-V curve of the case, where the voltage appears, asymptotically approaches the straight line that passes the origin of the coordinate system. This intrinsic I-V characteristic of the Josephson junction is called the RSJ (Resistively Shunted Junction) characteristic.
In case the junction layer is too thin resulting in short circuiting, on the other hand, a voltage gradually appears when the current is higher than the critical current Ic as shown in FIG. 4C. This phenomenon is called the FF (Flux Flow) type I-V characteristic, since the voltage is induced by the movement of magnetic flux.
When the first and second superconductors are made of the same superconductor material as the example shown in FIG. 3, FF type I-V characteristic is likely to result since the first and second superconductors have high affinity with each other so that the first and second superconductors may be short circuited across the interface of the junction at some points if the conditions of forming the second superconductor are similar to those of the first superconductor.
Based on the above discussion, I-V characteristic of a junction may be made less likely to become FF type by forming the first and second superconductors from materials of different compositions, thereby causing the first and second superconductors to have less affinity with each other. When the first and second superconductors are formed from materials of different compositions, the superconductors have different intervals between CuO2 planes, that are major transmission paths in the high temperature superconductors, in the c-axis direction, so that the CuO2 planes of the first and second superconductors become less compatible to each other at the junction, and the junction should be less likely to have FF type I-V characteristics as shown in FIG. 5.
With this background, candidates for the material to form Josephson junctions extended from YBa2Cu3Oy to YBa1.95La0.05Cu3Oy where a part of Ba is replaced with a trace of La, and NdBa2Cu3Oy or YbBa2Cu3Oy where Y is replaced with Nd or Yb. Thus such combinations of second superconductor/first superconductor as YBa2Cu3Oy/NdBa2Cu3Oy, YbBa2Cu3Oy/YBa2Cu3Oy, Y(Ba, La)2Cu3Oy/YBa2Cu3Oy/YBa2Cu3Oy, and YBa2Cu3Oy/Y(Ba, La)2Cu3Oy have been tried.
However, satisfactory I-V characteristic of the junction could not necessarily be obtained from the combinations described above.
Moreover, producing Josephson junctions that have high reliability requires the use of high quality materials for both the first and second superconductors, as well as solving the problem of affinity between the first and second superconductor materials.
For this reason, when making a Josephson junction that requires a process of stacking a multitude of thin films, for example, care must be exercised to avoid adversely affecting the quality of the underlying first superconductor, that has been formed beforehand, during the process of forming the second superconductor. In other words, formation of the second superconductor thin film in a Josephson junction, particularly ramp-edge type Josephson junction, must be done under such conditions that do not degrade the first superconductor that has been formed beforehand.
However, a physical film forming process such as sputtering process or laser deposition process does not offer process conditions having large margins that allow it to form a c-axis orientation film that is good in all respects including superconductivity, surface flatness and crystallinity, which are related to achieving high quality of superconductor film.
As an example, in FIG. 6, a black bar indicates a range of substrate temperatures that allow it to form a c-axis orientation film of flat surface from LnBa2Cu3Oy (Ln═Nd, Sm and Eu, Gd, Dy, Ho, Er, Tm, Yb or Y) by the off-axis high frequency sputtering process.
Simply forming a c-axis orientation film may be possible over a wider range of temperatures, although tolerance is restricted to such a narrow range of temperatures as shown in FIG. 6, when it comes to forming a thin film of high quality with surface roughness Ra of 5 nm or less as measured with an atomic force microscope (AFM) as well as being visibly flat as viewed by an optical microscope.
Therefore, it is a very tedious task to find out the optimum film forming conditions for both the second superconductor and the first superconductor. Furthermore, operating conditions of thin film manufacturing apparatuses may differ from one apparatus to another, and there may be cases in which proper conditions for forming a c-axis orientation film of high quality cannot be found for the two superconductor materials that have been selected.
Accordingly, since it is nearly impossible to form thin films of the best quality for the first and second superconductors, the upper and lower electrodes are formed while seeking trade-off points, in the actual film forming process.
Thus, it is difficult to obtain a product having high performance and high reliability, even in the case of a Josephson junction formed by using the same material for both the first and second electrodes, and it is much more difficult to obtain a product having good quality in terms of superconductor films and high reliability, in the case of a Josephson junction formed by using different materials for the first and second superconductors.
The most important portion of a Josephson junction, ramp-edge type Josephson junction in particular, is a layer as thin as 1 to 2 nm where the first and second superconductors make contact, and reliability of the product device depends on establishing the technology to form this portion with good reproducibility. Actually, however, the conditions for reliably forming the desired junction have not been clarified.
Now, the second problem of the prior art will be described below. When process parameters are improperly set for forming a ramp-edge junction or stacked junction, short circuiting may occur across the junction layer at some points thereof as it becomes difficult to control the very small thickness of the junction layer. Current-voltage characteristics (I-V characteristic) of the junction vary depending on the thickness of the junction.
When the junction layer is too thick, superconducting current cannot flow and therefore the junction shows current-voltage characteristic (I-V characteristic) of a resistive material as shown in FIG. 4A.
When the junction layer has a proper thickness, superconducting current can flow by tunneling through the junction without causing a voltage within the range of critical current Ic as shown in FIG. 4B. As soon as the current exceeds the critical current Ic, a voltage appears across the junction. The I-V curve of the case, where the voltage appears, asymptotically approaches the straight line that passes the origin of the coordinate system. This intrinsic I-V characteristic of the Josephson junction is called the RSJ (Resistively Shunted Junction) characteristic.
In case the junction layer is too thin, resulting in short circuiting, on the other hand, voltage gradually appears when the current is higher than the critical current Ic as shown in FIG. 4C. This phenomenon is called the FF (Flux Flow) I-V characteristic, since the voltage is induced by the movement of magnetic flux.
Producing a superconducting electronic device that employs the Josephson effect requires it to form a number of Josephson junctions that show the RSJ characteristic described above and have appropriate values of critical current Ic and IcRn product. At present, however, there is significant variance among the Josephson junctions, and satisfactory levels of stability of characteristics and reliability have not been achieved.
In particular, since critical current Ic is sensitive to the junction structure and manufacturing process, there is an urgent need for a technology that restrains the variation in the critical current Ic.
In order to achieve industrial application of the Josephson junction, it is essential to establish a technology to manufacture a plurality of Josephson junctions, that operate with proper characteristics, with good reproducibility. To achieve large scale integration in the future, capability to manufacture a large number of Josephson junctions with smaller variance in the characteristics thereof is required. While research efforts have so far been focused on the Josephson junction that uses an oxide high temperature superconductor having a high superconducting transition temperature (Tc), with the aim of achieving a superconducting electronic device having higher operating temperature. However, verification of operation has been limited to small circuits because of significant variability in the characteristics.
According to J. Talvacchio et al, for example, in order to have a circuit consisting of 100 junctions or more to operate, variations in the characteristics represented in terms of the proportion (percentage) of standard deviation (σ) of the value of the characteristic in question to the mean value of the characteristic (X) must be controlled to within 10% (100 σ/X≦10) (J. Talvacchio et al., IEEE Trans. Appl. Supercond. 7, 2051 (1997)). A successful case of achieving this requirement for ramp-edge type Josephson junction has recently been reported.
Meanwhile Satoh et al. achieved variance (100 σ/X) of 8% at 4.2K for 100 junctions made by using YBa2Cu3Oy for the superconductor electrode and (La0.3Sr0.7)(Al0.65Ta0.35)Oy for the insulation layer (T.Satoh et al., IEEE Trans. Appl. Supercond. 9, 3141 (1999), Japanese Unexamined Patent Application, First Publication No.2000-150974 A).
According to Satoh et al., good junction characteristics are achieved due to a uniform barrier layer of thickness within 2 nm being formed between two superconductor electrodes and mixing of La that has diffused from the insulation layer into the interface during etching. But concentration of La that has mixed in is extremely low and cannot be detected by observing with an analytical type transmission electron microscope that uses characteristic X-rays generated by irradiating an electron beam about 1 nm in diameter (J. G. Wen et al., Appl. Phys. Lett. 75 (1999)).
Saotome et al. achieved a standard deviation (σ) of 7.9% at 4.2K for 100 junctions made by using YBa2Cu3Oy-x for the superconductor electrode and CeO2 for the insulation layer (Soutome et al., Proceedings of 62nd Academic Lecture Conference of The Japanese Applied Physics Society, No. 1, p. 195 14a-G-7 (Sep. 11–14, 2001)). They did not use any material that included La, and achieved the value described above with such a structure and a process in which La does not mix in between the two superconducting electrodes.
Conventional Josephson junctions that show a standard deviation (σ) of about 8% among 100 junctions have been made as described above, but these values are not sufficient to integrate over 100 junctions. Thus, there has been a demand for a type of Josephson junction that allows integration of a larger number with less variability in the characteristics.